E. coli is a gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms. Most E. coli strains are harmless. However, some strains are pathogenic and can cause serious illness in humans and other animals. Illnesses caused by pathogenic E. coli include, for example, gastrointestinal infections, skin infections, respiratory infections, urinary tract infections, neonatal meningitis, inflammation, septicemia, mastitis, colibacillosis, perihepatitis, pericarditis, and peritonitis.
In particular, avian pathogenic E. coli (APEC) is a group of E. coli strains that cause a variety of respiratory and skin diseases in chickens, turkeys, and other avian species. APEC are the most common bacterial pathogen in chickens, costing the poultry industry hundreds of millions of dollars in economic losses worldwide. The economic losses from colibacillosis, caused by APEC, arise from the increased mortality and decreased growth rate of the affected birds. For example, in Brazil, which is the world's largest exporter of chicken meat, APEC are responsible for 45.2% of condemned poultry carcasses. See Fallavena et al., Avian Pathol., 2000, 29:557-562, incorporated by reference in its entirety.
In addition to the economic losses, APEC isolates are suspected to be a major source for spreading antimicrobial resistance to other human and animal pathogens, mainly through their plasmids and the exchange of genetic material with other bacteria. Even in countries and regions with strict limits on antibiotic use in the poultry industry, such as the U.S., Australia, and Europe, up to 92% of avian E. coli isolates are resistant to three or more antimicrobial drugs. See Gyles et al., Anim. Health Res. Rev., 2008, 9:149-158, incorporated by reference in its entirety.
APEC are abundant on chicken farms, and inhalation of dust particles loaded with bacteria is the main route of infection. The disease develops quickly, within 24-48 hours, and can only be cured though the use of antimicrobial drugs. The short lifespan of meat-type chickens (37-40 days) makes using antibiotics a very unlikely scenario, as the infection typically occurs around 21-28 days of age and antimicrobial treatment requires a withdrawal period of at least 14 days before the birds are shipped off the farm. Moreover, increased use of antibiotics, due to APEC, contributes to the emergence of antibiotic-resistant strains of pathogenic E. coli. 
Accordingly prevention of APEC is a better option than treatment. Provided herein are vaccines that prevent APEC, and infection with other forms of pathogenic E. coli. Also provided herein are methods of using these vaccines to prevent infection of avians with APEC and infection of other subjects with pathogenic E. coli. 
While vaccines against APEC do exist, most APEC vaccines involve the use of whole microorganisms. The subunit vaccines described herein are advantageous in that they induce a more targeted and uniform immune response, and can be manufactured more consistently.